Wood
Luckily, the scrap piece of plywood I had was cabinet grade (finished
quality), which didn’t require much sanding. If you don't mind
working your fingers to the bone, buy construction grade and save a
few bucks.

Nice Legs
Buy ready-made legs at the hardware or lumber store. They should come
with a top plate and screws and are usually sold 4 to a pack.

Bright Idea: If you don’t have scraps of plywood lying around,
the lumberyard will. They also have precut circles or rectangles.
Just ask or dig around and you can find some great.